Radar level gauges (RLGs) are suitably used for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials. An example of such a radar level gauge can include a transceiver for transmitting and receiving microwaves, a propagation device arranged to direct microwaves towards the surface and to return microwaves reflected by the surface to the transceiver, timing circuitry adapted to control the transceiver and to determine the level based on a relation between microwaves transmitted and received by the transceiver and an interface arranged to receive power and to connect the radar level gauge externally thereof.
In a case where measurements are made in a tank containing explosive gas or liquids, or in any other situation where the radar level gauge is located in an explosion endangered area, it is required that the RLG is provided with explosion protection. This requirement can be fulfilled using an intrinsically safe (IS) design, in which a barrier is used to limit voltages, current and power supplied to the RLG. Such a barrier is generally placed at the input to the communication interface, thus protecting the complete system.
The propagation device, for example an antenna or a guided wave probe (i.e. transmission line suspended from top to bottom in the tank), also requires a ground reference. A problem in this context is that the RLG will also be connected to a ground reference through the communication interface, and a ground loop circuit can develop when the different ground references are tied to different potentials. Such a ground loop may add or subtract current or voltage from the measurement process, possibly distorting the measurement signal.
In order to avoid interference, disturbance or malfunction from multiple grounding, capacitors can be used to prevent DC current and the low frequency communication signal from interfering with the measurement signal. If the propagation device is operated at high frequency (e.g. microwaves) such capacitors can effectively disconnect the propagation device from the communication circuitry, avoiding problems associated with ground loops. An example of such a system is disclosed in U.S. Pat. No. 6,750,808.
However, separating the propagation device by means of capacitors can be a complex solution, especially when transmitting an unmodulated pulse, as often is the case when using guided wave radar. The reason is that the transmitted pulse in this case has a broad bandwidth, which cannot easily be separated from the communication signal. In addition, the total amount of capacitance in the RLG is limited due to intrinsic safety requirements.
When the propagation device is a free propagating antenna, the antenna is normally galvanically separated from the circuitry by means of an isolated feeder connected to a wave guide, and ground loops are thus not an issue. However, the issue of limited capacitance is still a problem.
In order to obtain a satisfactory signal to noise ratio, it is desirable to transmit electromagnetic signals with high power. In order to transmit waves of higher power in a situation when the available power is limited, such as when using a two-wire interface, the transceiver may use capacitors as energy storage to temporarily provide a greater power level. However, the total amount of capacitance in an intrinsically safe circuit is limited, thus potentially limiting the sensibility of the RLG.